ABSTRACT Humoral hypercalcemia of malignancy (HHM) is a common and deadly complication of cancer defined by a constellation of biochemical abnormalities that includes elevated calcium levels, suppressed PTH levels, low serum phosphorus, a decreased tubular maximum for phosphate reabsorption corrected for GFR (TmP/GFR), elevated nephrogenous cAMP (NcAMP) excretion and suppressed 1,25 (OH)2 vitamin D levels. HHM is caused by tumor cell secretion of parathyroid hormone-related protein (PTHrP), a peptide growth factor related to parathyroid hormone (PTH) that interacts with a shared PTH/PTHrP receptor in bone and kidney to mimic systemic actions of PTH. Even though PTH and PTHrP act on the same receptor, the biochemical profile caused by excess PTHrP in HHM differs from that caused by excess PTH in primary hyperparathyroidism. First, elevated PTHrP levels in HHM are associated with suppressed 1,25 (OH)2 vitamin D levels whereas elevated PTH levels in primary HPT are associated with increased 1,25 (OH)2 vitamin D levels. Second, while serum phosphate levels in the two conditions overlap, TMP/GFR levels in HHM are lower than they are in primary HPT. Lastly, in patients with HHM, bone resorption rates are elevated but bone formation rates are suppressed. In contrast, in patients with primary HPT, bone formation and bone resorption rates are both elevated. Although PTHrP was discovered almost 30 years ago, these differences have never been adequately explained. Furthermore, despite the introduction of new drugs to treat hypercalcemia, the prognosis in HHM remains as dismal as it was 30 years ago, underscoring the need to better understand this syndrome and to develop better treatments. In order to address these issues, we created MMTV-rtTA;pTet- PTHrP;MMTV-PyMT (Tet-PTHrP;PyMT) mice to serve as a tetracycline-regulated transgenic breast tumor model of HHM. Our preliminary observations demonstrate that, in these mice, HHM is associated with extremely high circulating levels of FGF23. This has led us to hypothesize that, in HHM, circulating PTHrP increases FGF23 secretion, and, in turn, FGF23 acts on the kidneys to reduce 1,25 (OH)2 vitamin D levels, on the skeleton to inhibit osteoblast function and on tumor cells to increase their proliferation. To test this hypothesis, we propose 3 Specific Aims. Aim 1 will explore whether overexpression of PTHrP in breast tumors Induces osteocyte FGF23 production. Aim 2 will explore whether inhibiting FGF23 function increases 1,25 (OH)2 vitamin D levels and bone formation rates in mice with HHM. Aim 3 will assess whether elevations in FGF23 contribute to tumor progression in HHM. These experiments are designed to explore the contribution of FGF23 to the biochemical signature of HHM and to provide a preliminary assessment of the utility of anti- FGF23 therapy in cancer patients with HHM.